The present invention relates to polyester films and magnetic recording media and film capacitors produced therefrom. More particularly, the present invention relates to polyester films containing particular silica particles, which is excellent in various properties, and the above products obtained from such polyester films.
Polyester films, especially biaxially stretched polyester films such as polyethylene terephthalate films have excellent physical, chemical and thermal properties and are widely used as fibers, films and other molded articles.
A film property which is generally desired in the production of these films and in handling of the produced films is good slipperiness between the films themselves or between the film and other base material. Poor slipperiness of film in the production process or the steps of aftertreatments such as coating and deposition or in handling of the product itself may lead to bad workability and reduced commercial value of the products.
For improving slipping properties of film, it has been generally practised to add fine particles into polyester to provide proper unevenness to the film surface.
As the fine particles used for such purpose, the particles of inorganic compounds inert to polyester such as talc, kaolin, silica, calcium carbonate, titanium dioxide, graphite, carbon black and the like have been employed. However, these inorganic compound particles, which are usually obtained by crushing natural minerals or by syntheses, are difficult to avoid the presence of coarse particles or agglomerates of particles.
If coarse particles are contained in polyester or dispersibility of particles is poor there tends to take place clogging of the filter in the extrusion step or a break of the film. Also, a defect such as fish-eye would be produced in the film, giving rise to the problems such as drop out in magnetic tapes or reduced dielectric strength in capacitors, etc.
Particularly in the field of magnetic tapes, requests for the reduction of size, longer play time and higher image quality are rising. For the attainment thereof, it is necessary to make the unevenness on the film surface more uniform and finer.
To this purpose, it has been proposed to remove coarse particles, before use, from the inert inorganic particles by crushing and classifying the particles. For example, a method is known in which a powder obtained by crushing starting mineral material or a powder obtained from a synthesis is subjected to a dry or wet crushing treatment and then to a dry or wet classifying treatment. This method, however, although capable of removing coarse particles to a certain extent, was still unsatisfactory.
Various types of silica particles differing in average diameter are commercially available. However, when films were made by incorporating such silica particles into polyester, the produced films would fail to meet said strict property requirements.
Generally, the finely divided particles having a relatively high surface activity such as silica particles tend to form agglomerate particles and can hardly be dispersed thoroughly in the disperse medium. Also, agglomeration of particles accelerates in the course of preparation of polyester after addition of the fine particles and, as a result, the coarse agglomerate particles are formed in polyester to make the unevenness on the film surface ununiform, resulting in a reduced commercial value of the produced film. Especially in case such film is used as support for magnetic tapes such as deposited tapes or metal tapes, a decrease of output or an increase of drop out of signals would be caused.
As a result of assiduous studies on dispersibility and possibility of forming coarse agglomerate particles of silica particles in the particle addition method, particularly in the method using silica particles as the particles to be added, the present inventors found that the amorphous fine spherical silica particles obtained by subjecting alkoxysilane to hydrolysis and a condensation reaction by using a specific catalyst have a very narrow particle size distribution, contain no coarse particles, have excellent dispersibility in media such as ethylene glycol and in polyester, and as a result, a polyester film containing such amorphous fine spherical silica particles has a fine and uniform surface roughness with minimized coarse protuberances and also having excellent flatness and slip characteristics. The present invention was attained on the basis of such finding.